DynamicRenderer::Load() can internally fall back from Vulkan to
OpenGL when the Vulkan library is missing or is_available() fails.
Previously RenderManager::backend_ stayed at kVulkan, so callers
asking RenderManager::backend() got the wrong answer. After a
successful dynamic load, sync backend_ from
DynamicRenderer::backend_name() and log any fallback.
ctest still passes 4/4 in build-default-on.
Mike-Solar
8b8aa945f0
